The m b : M, relation for explosions at the Nevada Test Site (NTS) differs from those for explosions in other parts of the world. There is considerable evidence that this results mostly from high body-wave attenuation in the upper mantle beneath the western US. The authors have developed an empirical magnitude correction for body-wave attenuation and applied it to both source and receiver ends of the teleseismic body-wave path. The results imply that mb values are lower for NTS explosions than for Soviet explosions of comparable yield and seismic coupling. The authors have also developed and applied a source-depth correction to account for pP-P interference in the P-wave arrival.The body-wave magnitude resulting from these corrections is designated WIQ to distinguish it from other definitions of mb. Values of m Q determined for a world-wide set of large explosions show that a single mQ: yield relation is a fair fit to the data for the explosions with high seismic coupling.However, grouping the explosions under two mQ : yield relations gives a better fit to the data. All the studied explosions in salt or granite or below the water table fit a common M, : yield relation. Explosions from North America, Eurasia and Africa have a common m Q : M, relation.

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In a recent paper Springer et al. [1968] present results concerning the Sterling nuclear decoupling experiment. At this time, I would like to present additional theoretical analyses verified with datg obtained over a wider frequency band. The additional data indicate that the alecoupling at low frequencies is somewhat less than that indicated by the data in the vicinity of 1 Hz. An analysis is performed that satisfactorily explains the observed frequency dependence in terms of a simple analytic expression. As shown in Figure 3 of the referenced paper, the seismic instruments used to record the events Salmon and Sterling are characterized by low-frequency rolloffs. In particular, the NC-21 velocity meter employed by the U.S. Coast and Geodetic Survey has a second-order rolloff at I Hz. Using an analog correction circuit [Watson, 1967], the inverse of this response function was programmed on an analog computer resulting in an effective instrument response that gives reliable information down to 0.3 Hz. We employed this circuit in the processing of the Salmon and Sterling measurements and thereby obtained data over the frequency range of 0.3-100 Hz. As mentioned by Springer et al. [1968], narrow bandpass filtering can be used to approximate Fourier analysis. We agree with this concept and routinely employ analog filters characterized by the following transfer function [Lynch, 1965]: + + S,o where $ is the Laplace transform variable, and •o is the center frequency of the filter. The

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Theoretical wave forms for the first cycle and a half are calculated for Romney's experimental recordings of underground nuclear explosions Blanca, Logan, and Tamalpais at distances of 96 to 714 km. Models of the crust are constructed from travel times. Zvolmskii s near‐front approximation is used to form the basis of amplitude calculations which include head coefficients, geometrical spreading, and corrections for superposed layers. The source function is scaled from measurements made of the Rainier shot. The effects of attenuation and instrument response are included. By convolving these factors, theoretical displacement amplitudes are calculated in millimicrons for the first half‐cycle which agree with the experimental measurements of Logan and Bianca from 300 to 600 km within +4 to −16 per cent. A single‐layer crustal model with a Q of about 400 is indicated by the amplitude calculations. The amplitudes of later half‐cycles are influenced by the reflection or interaction at the surface of the Rainier mesa. Additional data and calculations indicate that the surface reflection or interaction is nonlinear and has an amplitude about three times that expected on an elastic basis.

An analysis is given of an experiment designed to test the theory of seismic decoupling of underground explosions proposed by Latter, LeLevier, Martinelli, and McMillan [1961]. The amplitude of the seismic signal from a 1.7‐kiloton nuclear explosion in a hole in salt was calculated and compared with the measured value from the 1.7‐kt Rainier shot in tuff at the same distance. A decoupling factor of about 300 resulted. The experiment, called Cowboy,1 was designed to test the decoupling principle by carrying out a series of eight high‐explosive shots in two spheres made in a salt dome, and nine tamped shots for comparison. The seismic data reported here were obtained primarily at ranges of 14,000 and 22,000 feet and at frequencies of 10 to 30 cps. A salt‐to‐salt decoupling factor of 100 was obtained which is consistent with the predicted tuff‐to‐salt factor of 300. When the sphere was overdriven so that the walls did not move elastically (which violates a condition of the theory for full decoupling), decoupling factors of 10 and 30 were measured. The seismic data are interpreted to give the dependence of decoupling on the various parameters of the experiment. The decoupling deduced from measurements made 80 feet from the shot points is found to be consistent with that deduced from the measurements at 14,000 and 22,000 feet. Project Cowboy was sponsored by the Atomic Energy Commission and carried out under the technical direction of the Lawrence Radiation Laboratory, University of California.

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Theoretical calculations of the first half-cycle amplitudes of the P wave at 250 to 1000 km are made from source functions representing several types of nuclear explosions in salt. Specifically, they are (1) Gnome scaled to 5 kt; (2) Salmon; (3) theoretical 25 kt detonated in a cavity built to fully decouple a 5-kt explosion; and (4) theoretical 5 kt also detonated in a 5-kt cavity. The calculations consist of a series of convolutions of the source functions plus the attenuation operators plus the instrument response. Amplitude factors are calculated using Zvolinskii's near-front approximation for various assumed crustal models. Experimental amplitudes from Salmon fall below the amplitude curve for the scaled Gnome source. They are consistent with amplitudes for the Salmon source; therefore, head-wave propagation along the M discontinuity may occur in the eastern United States. This is not conclusive, however, since body-wave calculations might give amplitudes of equal consistency. Amplitudes calculated for the fully decoupled 5-kt source show a decoupling of 200. Those from the overdriven cavity source indicate that a partial decoupling of 80 can be gained by overdriving a cavity by a factor of 5. INTRODUTION T•E SOURCEThere has been some contention that no true head wave from the M discontinuity would be seen as a first arrival from the Salmon event.This argument was based on the apparent differences in the earth's crust in the eastern continental United States from those in the west.